The present invention relates to noise eliminating apparatus and method of a magnetic disk unit for preventing that noises are mixed into a read signal which is read out from a disk medium and is transmitted to an upper apparatus and, more particularly, to noise eliminating apparatus and method of a magnetic disk unit for preventing that noises which are generated by the operation of a data bus for connecting a drive unit with an upper apparatus are mixed into a read signal.
In a magnetic disk subsystem which is used as an external memory apparatus of a computer, one or a plurality of drive units are connected under the domination of an upper disk controller. As such a subsystem, there are a concentrated processing type and a distributed processing type. In the concentrated processing type, a formatter is provided for the upper controller and, in the reading mode, an analog read signal is generated from the drive unit, thereby reconstructing read data on the controller side. In the distributed processing type, a formatter is provided for each drive unit and all of the read data is reconstructed by the drive unit and is transmitted to the upper apparatus. The present invention belongs to the former concentrated processing type.
In the case where the formatter section is provided for the upper controller, when receiving a seek command from the upper controller, the drive unit moves a head to a designated cylinder address by the driving of an actuator by a voice coil motor. The actuator for positioning the head is generally controlled by a micro processor or a digital signal processor.
When the drive unit notifies the completion of the seeking operation to the upper controller, an analog write signal is transmitted by a write signal line in the writing mode and is supplied to a write head and is written into a disk medium. In the reading mode, when receiving a confirmation notification from the upper controller for the seek completion response, a read signal of a read head (MR head) is amplified by a reading amplifier and is transmitted to the upper controller by a read signal line.
FIG. 1 is a schematic diagram of a conventional magnetic disk drive unit comprising: a disk enclosure 100; a reading amplifier board 102; and a drive analog board 104 on which a processor 106 is installed. The disk enclosure 100 is a mechanism portion having a disk medium, a head, an actuator, a voice coil motor, a spindle motor, and a head IC. A reading amplifier with an AGC function is installed on the reading amplifier board 102. In addition to the processor 106, a D/A converter, an A/D converter, a servo demodulating circuit, and the like are installed on the drive analog board 104. The processor 106 on the drive analog board 104 is connected with the upper controller by a read signal line 108, a write signal line 109, and a data bus 110 and transmits and receives various kinds of commands, the write signal, and the read signal.
The head provided for the disk enclosure 100 integrally supports a write head and a read head at the edge of the actuator. Hitherto, a magnetic head has been used as each head. On the other hand, in recent years, a small MR head using a magnetoresistive device is used as a read head in order to improve a track recording density. The MR head can be miniaturized as compared with the magnetic head and also has a high magnetic converting efficiency. Assuming that those heads have the same intensity of magnetic field, the MR head generates a read voltage higher than that of the magnetic head. In order to make the MR head operative as a magnetoresistive device, it is necessary to supply a predetermined bias current in the reading mode. For this purpose, a bias voltage of about 2.0 V is supplied from the drive analog board 104 to the disk enclosure 100 via a bias signal line 112, thereby applying the bias voltage to the MR head. Further, when the MR head to which the bias voltage was applied is come into contact with the disk medium, an excessive short-circuit current flows, thereby causing a head breakage. Therefore, by also supplying the same bias voltage to both of the disk medium and the casing side, they are set to the same potential and a head breakage is prevented.
The realization of a miniaturization and a high density of the magnetic disk unit is remarkable in recent years. The inventors of the present invention has examined to miniaturize the drive unit of FIG. 1, so that a drive unit in which an area of a printed circuit board is reduced to about the half could be developed as shown in FIG. 2. In this case, in addition to the miniaturization of a disk enclosure 114, a reading amplifier 118 is installed on a drive analog board 116 together with a processor 120.
In the miniaturized drive unit in FIG. 2, however, there occurs a problem such that fairly large noises 122 are periodically mixed to a read waveform 124 in FIG. 3B which is generated in an active state of a read gate in FIG. 3A in the upper controller, so that read data cannot be accurately reconstructed. By examining a generation source of the noises 122 which are mixed to the read waveform 124, it has been found that the noises were generated synchronously with the switching operation of a data bus in FIG. 3C. Such noises of the read waveform don.varies.t cause a problem in the drive unit in FIG. 1 and appear for the first time by remarkably miniaturizing as shown in FIG. 2.
The first cause of the mixture of the noises is that the bias voltage is applied to the MR head. The bias voltage of about 2.0 V is applied from the drive analog board 116 to the MR head of the disk enclosure 114. In association with the miniaturization of the circuit board, the noises generated by the operation of the data bus are mixed to the bias voltage generated in the bias voltage generating circuit and are supplied to the MR head, so that the noises appear in the read signal. Although an output voltage of the MR head is generally equal to about 0.5 mV, noises of about 50 mV which is 100 times as large as such an output voltage are mixed according to the actual measurement. A head IC provided for the disk enclosure 114 uses a differential circuit to prevent noises. However, in case of such large noises of 50 mV, a difference between noises mixed to two differential signal lines appears in an output.
The second cause of the mixture of noises is that since the area of the printed circuit board is reduced, a distance between the read signal line 108 and the data bus 110 decreases and noises by crosstalk are easily mixed. Since the noises by the crosstalk increase in proportion to the square of the distance between the signal lines, it is considered that such a short distance becomes a cause.